How does water travel from roots to leaves?
Follow the starred parcel. Water from moist soil crosses tiny root hairs and enters the stem’s water-carrying channels, called xylem. Root hairs provide more contact with soil; xylem connects the route to the leaves.
An important upward pull starts in the leaves. Water evaporates from wet cell surfaces and passes through stomata into the air. As water leaves, tension in the continuous column helps draw replacement water upward. Water molecules attract one another and cling to wet channel walls, helping the pathway stay continuous.
Follow this route: soil supplies water, xylem connects the path, and evaporation at the leaves provides a pull. The star helps track a parcel; water molecules are not a rigid rope.
Bag a leaf
- Ask an adult to slip a clear, initially dry bag over a healthy leafy branch. Tie it gently without damaging the stem, and keep wet soil out of the bag.
- Hang an identical empty bag nearby as a control. Keep both in the same light, away from strong direct sun.
- Compare after an hour or two, then remove the bag. Which has more droplets? Water vapor lost from leaves condenses on the bag into visible drops.
Bagging itself raises humidity and can change temperature, so this does not directly measure the effect of outdoor humidity. Use the model to change humidity alone while holding other conditions fixed.
Water potential, resistance and limits
Open: read the flow like a circuit
Soil, root, xylem and leaf act like four hydraulic resistances in series:
The leaf side is “sucked” by transpiration: the bigger the vapour pressure deficit and the wider the stomata, the larger E. At steady state Ψ_leaf = Ψ_soil − E·R_total. Drier air or drier soil drags the leaf potential lower.
Open: the stomatal dilemma
Stomata are adjustable gates: wide open brings in carbon dioxide and speeds photosynthesis, but also lets water escape. Plants constantly trade “saving water” against “growing”. In drought, stomata narrow and the upward stream slows—the very effect you drive with the opening slider.
Limits of the model
Capillary rise depends on gap size. In tall trees, transpiration-driven tension and water cohesion are the main transport mechanism. The model excludes root pressure, active salt transport, embolisms (cavities that break the column) and sugar transport in phloem; the parcel is a fluid marker, not a claim that water molecules chain rigidly together.